1. Field of the Invention
The present invention relates to a heat-conductive aluminum nitride sintered body and a method of manufacturing the same, and more particularly, it relates to a dense heat-conductive aluminum nitride sintered body which has excellent characteristics such as heat conductivity, insulability and permittivity, and a method of manufacturing the same.
2. Description of the Background Art
Technique relating to a large scale integrated circuit (LSI) has been remarkably advanced in recent years, particularly with improvement in circuit density. Increase in chip size of a semiconductor integrated circuit (IC) also contributes to such improvement in circuit density, and calorific power of a package for carrying an IC chip is increased with such increase in IC chip size. Thus, importance has been attached to heat radiation of a material for an insulator substrate, which is applied to a package for a semiconductor device or the like. Such an insulator substrate is generally prepared from alumina (Al.sub.2 O.sub.3). However, with small thermal conductivity of 30 W/mK, alumina is inferior in heat dissipation in contrast to excellence in electrical insulability and mechanical strength, and hence it is inadequate to carry a field-effect transistor (FET) of high calorific power or the like on an alumina substrate. Although there is an insulator substrate which is prepared from beryllia (BeO) having high thermal conductivity for carrying a semiconductor element of high calorific power, safety measures for using such a substrate are complicated since beryllia is toxic.
With respect to such an insulator substrate for carrying a semiconductor element of high calorific power, therefore, heat-conductive aluminum nitride (AlN), which is intoxic and equivalent in electrical insulability and mechanical strength to alumina, has been recently watched as an effective insulating material for a semiconductor device or a package.
As hereinabove described, aluminum nitride theoretically has high thermal conductivity and insulability as a monocrystal material. However, since powder of aluminum nitride itself is inferior in degree of sintering, relative density of a sintered body prepared by forming such aluminum nitride powder and sintering the same is about 70 to 80% at the most with reference theoretical density of aluminum nitride of 3.26 g/cm.sup.3, depending on sintering conditions, while the sintered body contains a large amount of pores. Therefore, it is independently employing aluminum nitride powder.
In an insulating ceramics member such as an aluminum nitride sintered body, thermal conduction is performed mainly through phonon conduction. Thus, phonon scattering is caused by defects such as pores, impurities etc. contained in the sintered body, to reduce the level of its thermal conductivity. In order to obtain an aluminum nitride sintered body having high thermal conductivity, various proposals have been made as follows:
(1) Japanese Patent Laying-Open Gazette No. 96578/1985, for example, discloses a method of adding Y.sub.2 O.sub.3, serving as a sintering assistant and a deoxidizer, to aluminum nitride powder and sintering the same. PA1 (2) Each of Japanese Patent Laying-Open Gazettes Nos. 71576/1985 and 155263/1986, for example, discloses a method of adding carbon to aluminum nitride powder and sintering the same for deoxidation, thereby to obtain a sintered body having a small content of oxygen. PA1 (3) Each of Japanese Patent Laying-Open Gazette Nos. 71575/1985 and 127267/1985, for example, discloses a method of employing high purity aluminum nitride powder having a small content of oxygen. PA1 (4) Japanese Patent Laying-Open Gazette No. 41766/1987, for example, discloses a method of decomposing/evaporating a sintering assistant remaining in a sintered body, to obtain an aluminum nitride sintered body of high purity. PA1 (5) Proceeding for 1987 Yogyo Kyokai's Annual Meeting p. 969, for example, discloses a method of removing a sintering assistant remaining in a sintered body by exposing the same to a reducing atmosphere for a long time.
In the method (1) of adding Y.sub.2 O.sub.3 and performing ball-mill mixing, it is necessary to add at least 1 percent by weight of Y.sub.2 O.sub.3 in order to obtain a dense heat-conductive sintered body. However, thermal conductivity of a sintered body thus obtained is about 200 W/mK at the most, and dispersion thereof is significant. If a large amount of Y.sub.2 O.sub.3 is added, thick intergranular phases are formed around aluminum nitride particles, to reduce oxidation resistance of the sintered body while increasing permittivity.
The method (2) is directed to reduce the amount of oxygen contained in aluminum nitride through a deoxidizing action of carbon. However, when no sintering assistant is added, the sintered body has low thermal conductivity of about 80 W/mK since it is difficult to densify the sintered body having low density, as disclosed in Japanese Patent Laying-Open Gazette No. 71576/1985. If Y.sub.2 O.sub.3 is to be added as a sintering assistant as disclosed in Japanese Patent Laying-Open Gazette No. 155263/1986, it is necessary to add at least 1 percent by weight of Y.sub.2 O.sub.3 in order to densify the sintered body.
According to Japanese Patent Laying-Open Gazette No. 71575/1985 listed in the item (3), the amount of oxygen contained in the sintered body is at least 0.5 percent by weight while the sintered body has low thermal conductivity of about 40 W/mK, even if aluminum nitride powder of high purity is employed as a raw material. As shown in Japanese Patent Laying-Open Gazette No. 127267/1985, further, thermal conductivity of a sintered body obtained by adding at least 3 percent by weight of Y.sub.2 O.sub.3 is about 70 W/mK.
As hereinabove described, an aluminum nitride sintered body obtained by adding a small amount of sintering assistant has low thermal conductivity in the existing circumstances.
According to the method (4) or (5) of adding a large amount of sintering assistant, sintering the material and thereafter removing the sintering assistant, on the other hand; it is assumed that an aluminum nitride sintered body thus obtained has high thermal conductivity exceeding 200 W/mK with improvement in permittivity etc. However, according to a method of removing the sintering assistant by adding a fluoride, a sintering furnace is inevitably contaminated and dispersion in characteristic of a sintered body thus obtained is increased not only in a lot but in the same sintered body, as shown in Japanese Patent Laying-Open Gazette No. 41766/1987. Further, the face of the sintered body is so roughened that after-treatment is inevitably required for employment, because of irregularity caused by evaporation/volatilization of the sintering assistant. According to the method of sintering the material in a reducing atmosphere for a long time as disclosed in Proceeding for 1987 Yogyo Kyokai's Annual Meeting, the cost required for such sintering is extremely increased while the sintering assistant segregates in the sintered face of the sintered body. Thus, the sintered body cannot be directly employed with such a sintered face, while the same has large dispersion in characteristic.